Why is it an oversimplification to call the pituitary gland the "master" gland?
Because it is under control of the hypothalamus.
The pituitary gland is often called the master gland because it produces hormones that control other hormone glands such as thyroid gland and the adrenal glands. However, it doesn't decide on its own whether or not to produce hormones and in what amount.
The hypothalamus is an important part of the brain that also produces hormones. A large part of these hormones are released directly (in)to the pituitary gland. These hypothalamic hormones stimulate or inhibit hormone production by the pituitary gland.
The hormones produced in the pituitary gland are then released into the general circulation from where the hormones reach their target organs. These organs / glands can send feedback signals to the hypothalamus and the pituitary gland which also stimulates or inhibits hormone production.